Degeneracy Temperature Research Articles

Effects of Interfacial Parameters and Temperature on Effective Schottky Barrier Height of Metal-Wrapped (Al-Gan) Nanowire Schottky Diode

Junction interface parameters (interface states density, interface layer thickness, interface permittivity and neutral surface states) and temperature effects have been studied. Results have shown that for the scaled structure doping functions below degeneracy state and temperature have significant impact on the effective Schottky barrier height while the interface parameters show no significant change. The barrier height has been observed to increase as the temperature increases almost linearly by ~5.171×10-3eV with increase in temperature from 300K to 420 K and this may be associated with the avalanched charge carriers being transported across the barrier height. The interface layer thickness in the range of 1.5 nm to 3.0 nm shows no effect on the carrier transport characteristics curves of the Schottky diode whereas the ideality factor has been found to decrease with an increase in temperature.

Influence of Photon Subsystem on the Magnetic Properties of Quantum Gases

The possibility for the photon component to affect the magnetic properties of a system of quantum gases of two-level atoms staying in thermodynamic equilibrium with radiation (photons) has been studied. A corresponding simple model has been proposed, which enabled the general equations describing the thermodynamic equilibrium in this system to be derived. The resulting equations are solved in the temperature interval far from the degeneracy temperatures of all three system components. The analysis of the solutions testified to a non-trivial behavior of the system’s magnetic state as a response to changes in the photon density and the intensity of the external magnetic field. It is shown that the growth of the photon density induced in the system by external sources can increase both system’s magnetization and the density of excited atoms. Such a conclusion is not trivial a priori given the fact that photons in the vacuum have no magnetic moment.

Direct Visualization of Dark Interlayer Exciton Transport in Moiré Superlattices.

Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide heterostructures are expected to achieve high-temperature exciton condensation. However, the exciton degeneracy state is usually optically inactive due to the finite momentum of interlayer excitons. Experimental observation of dark interlayer excitons in moiré potentials remains challenging. Here we directly visualize the dark interlayer exciton transport in WS2/h-BN/WSe2 heterostructures using femtosecond transient absorption microscopy. We observe a transition from classical free exciton gas to quantum degeneracy by imaging temperature-dependent exciton transport. Below a critical degeneracy temperature, exciton diffusion rates exhibit an accelerating downward trend, which can be explained well by a nonlinear quantum diffusion model. These results open the door to quantum information processing and high-precision metrology in moiré superlattices.

Metastable States of a Fluid Inside a Binodal in the Context of Cluster Variation

The behavior of the isotherm and molecular distributions inside the binodal is analyzed to solving an Ising model obtained on the basis of cluster variation for planar lattices with coordination numbers 3, 4, 6. It is found that the microscopic approach gives a probabilistic interpretation of the Maxwell macroscopic rule and explains how the isotherm a secant appears between the regions of coexistence of two phases. A region of no solutions (the region of degeneracy) is found inside the binodal, and the critical temperatures of degeneracy at which the nontrivial solution to the equations disappears are calculated for this region. The region of degeneracy inside the binodal expands and approaches the binodal curve as the temperature falls, so the degeneracy curve and the binodal become indistinguishable. Numerical iterative calculations are used to study the dependence of the region of no solution inside the binodal as a cluster grows. The critical temperature of degeneracy asymptotically approaches that of the binodal as the cluster grows. Existing ways of interpreting metastable states are discussed, along with as the correspondence between the new results and previously known mean-field (ignoring correlations) and quasi-chemical (considering only direct correlations) approximations, and an exact result of the Yang–Lee condensation theory.

Thermal resistivity and hydrodynamics of the degenerate electron fluid in antimony

Detecting hydrodynamic fingerprints in the flow of electrons in solids constitutes a dynamic field of investigation in contemporary condensed matter physics. Most attention has been focused on the regime near the degeneracy temperature when the thermal velocity can present a spatially modulated profile. Here, we report on the observation of a hydrodynamic feature in the flow of quasi-ballistic degenerate electrons in bulk antimony. By scrutinizing the temperature dependence of thermal and electric resistivities, we detect a size-dependent departure from the Wiedemann-Franz law, unexpected in the momentum-relaxing picture of transport. This observation finds a natural explanation in the hydrodynamic picture, where upon warming, momentum-conserving collisions reduce quadratically in temperature both viscosity and thermal diffusivity. This effect has been established theoretically and experimentally in normal-state liquid 3He. The comparison of electrons in antimony and fermions in 3He paves the way to a quantification of momentum-conserving fermion-fermion collision rate in different Fermi liquids.

Critical point for Bose–Einstein condensation of excitons in graphite

An exciton is an electron-hole pair bound by attractive Coulomb interaction. Short-lived excitons have been detected by a variety of experimental probes in numerous contexts. An excitonic insulator, a collective state of such excitons, has been more elusive. Here, thanks to Nernst measurements in pulsed magnetic fields, we show that in graphite there is a critical temperature (T = 9.2 K) and a critical magnetic field (B = 47 T) for Bose-Einstein condensation of excitons. At this critical field, hole and electron Landau subbands simultaneously cross the Fermi level and allow exciton formation. By quantifying the effective mass and the spatial separation of the excitons in the basal plane, we show that the degeneracy temperature of the excitonic fluid corresponds to this critical temperature. This identification would explain why the field-induced transition observed in graphite is not a universal feature of three-dimensional electron systems pushed beyond the quantum limit.

T-square resistivity without Umklapp scattering in dilute metallic Bi2O2Se

Fermi liquids (FLs) display a quadratic temperature (T) dependent resistivity. This can be caused by electron-electron (e-e) scattering in presence of inter-band or Umklapp scattering. However, dilute metallic SrTiO3 was found to display T2 resistivity in absence of either of the two mechanisms. The presence of soft phonons as possible scattering centers raised the suspicion that T2 resistivity is not due to e-e scattering. Here, we present the case of Bi2O2Se, a layered semiconductor with hard phonons, which becomes a dilute metal with a small single-component Fermi surface upon doping. It displays T2 resistivity well below the degeneracy temperature in absence of Umklapp and inter-band scattering. We observe a universal scaling between the T2 resistivity prefactor (A) and the Fermi energy (EF), an extension of the Kadowaki-Woods plot to dilute metals. Our results imply the absence of a satisfactory understanding of the ubiquity of e-e T2 resistivity in FLs.

Condensation of indirect excitons

Abstract

Possible nuclear fusion of deuteron in the cores of Earth, Jupiter, Saturn, and brown dwarfs

Many brown dwarfs have recently been discovered as sub-stellar objects in which deuteron thermonuclear fusion is taking place. Although Jupiter and Saturn emit nearly twice as much heat as they absorb from the Sun, their internal heat-generation mechanisms have been determined to differ from the nuclear fusion that fuels brown dwarfs because they have a mass factor of 0.023–0.077 less than that of brown dwarfs. The possibility for deuteron nuclear fusion in the Earth’s core has not been well studied. Here, we compare the conditions for electron degeneracy pressure and temperature for the cores with an Fe–D compound of Earth, Jupiter, and Saturn to the core with deuterium gases of the coldest brown dwarf, WISE 1828+2650, in respect to three-body deuteron nuclear fusion, based on electron capture and internal conversion processes. Our results suggest that deuteron nuclear fusion is possible in the cores of Earth, Jupiter, and Saturn as well the coldest brown dwarf.

Indirect Excitons and Trions in MoSe2/WSe2 van der Waals Heterostructures.

Indirect excitons (IX) in semiconductor heterostructures are bosons, which can cool below the temperature of quantum degeneracy and can be effectively controlled by voltage and light. IX quantum Bose gases and IX devices were explored in GaAs heterostructures where an IX range of existence is limited to low temperatures due to low IX binding energies. IXs in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by large binding energies giving the opportunity for exploring excitonic quantum gases and for creating excitonic devices at high temperatures. TMD heterostructures also offer a new platform for studying single-exciton phenomena and few-particle complexes. In this work, we present studies of IXs in MoSe2/WSe2 heterostructures and report on two IX luminescence lines whose energy splitting and temperature dependence identify them as neutral and charged IXs. The experimentally found binding energy of the indirect charged excitons, that is, indirect trions, is close to the calculated binding energy of 28 meV for negative indirect trions in TMD heterostructures [Deilmann, T.; Thygesen, K. S. Nano Lett. 2018, 18, 1460]. We also report on the realization of IXs with a luminescence line width reaching 4 meV at low temperatures. An enhancement of IX luminescence intensity and the narrow line width are observed in localized spots.

Exchange-correlation contributions to thermodynamic properties of the homogeneous electron gas from a cumulant Green's function approach

Thermodynamic properties of the interacting homogeneous electron gas are calculated using a finite-temperature cumulant Green's function approach over a broad range of densities and temperatures up to the warm dense matter regime $T\ensuremath{\sim}{T}_{F}$, where ${T}_{F}$ is the Fermi degeneracy temperature. These properties can be separated into independent particle and exchange-correlation contributions, and our focus here is on the latter. Our approach is based on the Galitskii-Migdal-Koltun and electron number sum rules from the finite temperature many-body Green's function formalism, together with an extension of the cumulant Green's function to finite temperature. Previously this approach yielded exchange-correlation energies and potentials in good agreement with quantum Monte-Carlo calculations. Here the method is extended for various thermodynamic quantities including the chemical potential, total energy, Helmholtz free-energy, electronic equation of state, specific heat, and isothermal compressibility, which optionally include spin dependence. We find that the exchange-correlation contributions are weakly varying at low temperature but exhibit significant temperature dependence in the WDM regime, as well as a crossover from exchange- to correlation-dominated behavior. In contrast to the $T={0}^{+}$ limit, we also find that renormalization effects are largely but not completely suppressed at finite temperature. Comparisons with other approaches at various levels of approximation are also discussed.

Wavefunction of plasmon excitations with space charge effects

The one dimensional (1D) driven quantum coupled pseudoforce system governing the dynamics of collective Langmuir electron oscillations is used in order to investigate the effects of variety of space charge distributions on plasmon excitations of a nearly free electron gas with an arbitrary degree of degeneracy and electron fluid temperature. A generalized closed form analytic expression for the grand wavefunction of collective excitations in the presence of an arbitrary space charge distribution is presented based on the stationary solutions of the driven coupled pseudoforce system which has been derived from the Schrödinger-Poisson model. The wavefunction and electrostatic potential profiles for some special cases such as the Heaviside charge distribution, Dirac charge sheet, impurity charge sheet in the 1D plasmonic lattice, and the Kroning-Penney Dirac charge distributions with particular applications in plasmonics and condensed matter physics are investigated in this paper. It is remarkably found that two parallel Dirac charged sheets completely shield all interior plasmon excitations with any given energy value from outside electrostatic fields and charge densities. It is also found that the presence of even a weakly charged impurity layer within a perfect 1D plasmonic crystal profoundly alters the periodic electrostatic field of the crystal lattice, and hence, the Bloch character of the wavefunction is considered in the bandgap theory of solids. The current investigation of electron excitations in arbitrary degenerate electron gas in the presence of static charge distributions may be used to develop analytic models for a variety of real physical situations. It also helps in further developments of the rapidly growing fields of nanotechnology and plasmonics.

Renormalization of the exciton mass in monolayer transition metal dichalcogenides

The renormalization of the total effective mass of exciton in monolayer transition metal dichalcogenides (TMDC) on different polar substrates are investigated arising from the exciton-optical phonons coupling, in which both the intrinsic longitudinal optical phonon modes and surface optical phonon modes induced by the polar substrate are taken into account. We find that the total effective mass of exciton is enhanced obviously due to these coupling, depending on the cut-off wave vector of optical phonon modes, the polarizability of material and the internal distance between the monolayer TMDC and the polar substrate. The renormalization effect on the exciton mobility and the critical temperature of quantum degeneracy of the dilute exciton gas are discussed. Our theoretical results provide insight for the analysis of the exciton mass in experiments in two-dimensional materials.

Drift KdV and KP equation in electron–positron–ion plasma with trapping effects

Linear and nonlinear propagation of one- and two-dimensional drift ion acoustic waves with the effects of trapping in electron–positron–ion degenerate plasma has been considered. The characteristics of the dispersion relation in a two-dimensional quantum degenerate plasma has modified significantly due to the inclusion of positrons. The variations of drift KdV and KP equation have been studied with magnetic field, positron concentration and electron degeneracy temperature. The system under consideration admits compressive solitary structures only. The present studies may be valuable to understand the propagation of drift solitary structures with special reference to white dwarf stars.

Polarized cold cloud of thulium atom

Minimization of internal degrees of freedom is an important step in the cooling of atomic species to degeneracy temperature. Here, we report on the loading of 6 × 105 thulium atoms optically polarized at maximum possible magnetic quantum number state into dipole trap operating at 532 nm. The purity of polarizations of the atoms was experimentally verified using a Stern–Gerlach-type experiment. Experimental measured polarization of the state is

Exchange and correlation in finite-temperature TDDFT

We discuss the finite-temperature generalization of time-dependent density functional theory (TDDFT). The theory is directly analogous to that at temperature T = 0. For example, the finite-T TDDFT exchange-correlation kernel fxc(T, n) in the local density approximation can again be expressed as a density derivative of the exchange correlation potential fxc(T, n) = [∂vxc(T, n)∕∂n]δ(r − r′), where n = N∕V is the electron number density. An approximation for the kernel fxc(T, n) is obtained from the finite-T generalization of the retarded cumulant expansion applied to the homogeneous electron gas. Results for fxc and the loss function are presented for a wide range of temperatures and densities including the warm dense matter regime, where T ≈ TF, the electron degeneracy temperature. The theory also permits a physical interpretation of the exchange and correlation contributions to the theory.

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.

Effect of degeneracy temperature on drift solitary structures in a nonuniform degenerate plasma

Linear and nonlinear propagation of quantum drift ion acoustic waves have been studied in a spatially inhomogeneous degenerate quantum plasma taking into account the effect of electron trapping. The linear dispersion relation has been derived in this regard in a 2D quantum magnetoplasma with trapped electrons and a significant amendment in the linear propagation characteristics of the wave under consideration has been observed due to the degeneracy temperature. The drift ion solitary structures have been investigated in 1D and 2D by employing the drift approximation. We obtained the compressive drift solitary structures which are analyzed numerically for the parameters, such as degeneracy temperature, typically found in white dwarfs.

Kondo screening and beyond: An x-ray absorption and dichroism study ofCePt5/Pt(111)

We use x-ray absorption spectroscopy as well as its linear and circular magnetic dichroisms to characterize relevant interactions and energy scales in the surface intermetallic CePt$_5$/Pt(111). The experiments provide insight into crystal field splitting, effective paramagnetic moments, their Kondo screening and mutual interactions and thus into many aspects which typically determine the low temperature behavior of correlated rare earth compounds. Exploiting the tuneability of Ce valence through the thickness dependent epitaxial strain at the CePt$_5$/Pt(111) interface, we are able to systematically investigate the impact of hybridization strength on these interactions. Considerable Kondo screening is indeed observed at all CePt$_5$ thicknesses, and found to be strongest in case of strongest hybridization. While the magnetic response is commensurate with an impurity Kondo scale of $T_K \gtrsim 10^2$ K for specimen temperatures $T \gtrsim 30$ K, this is no longer the case at lower temperature. Its detailed study by XMCD at one specific thickness of CePt$_5$ reveals an anomaly of the susceptibility at $T^* \approx 25$ K instead, which we tentatively associate with the onset of lattice coherence. At lowest temperature we observe paramagnetic saturation with a small Ce $4f$ saturation magnetization. Within the framework of itinerant $4f$ electrons, saturation is due to a field induced Lifshitz transition involving a very heavy band with correspondingly small degeneracy temperature of $T_F \approx 7$ K. This small energy scale results in the persistence of Curie-Weiss behavior across the entire range of experimentally accessible temperatures ($T \gtrsim 2$ K). Our work highlights the potential of magnetic circular dichroism studies in particular for Kondo and heavy fermion materials, which so far has remained largely unexplored.

Collective phenomena in cold indirect excitons

Due to their long lifetimes, indirect excitons can cool to below the temperature of quantum degeneracy. This gives an opportunity to experimentally study cold composite bosons. Both theoretically predicted phenomena and phenomena that have not been anticipated were observed in a cold gas of indirect excitons. In this contribution, we overview our studies of cold indirect excitons over the past decade, presenting spontaneous coherence and condensation of excitons, spatially modulated exciton state, long-range spin currents and spin textures, and exciton localization–delocalization transitions.